cmDefinitions: Make ClosureKeys API vector-based.

Construct the final list directly in a named return value.  Use
a single set to track bindings already found.

Co-Author: Brad King <brad.king@kitware.com>

Source/cmDefinitions.h

@@ -50,7 +50,7 @@ public:
   cmDefinitions Closure() const;
 
   /** Compute the set of all defined keys.  */
-  std::set<std::string> ClosureKeys() const;
+  std::vector<std::string> ClosureKeys() const;
 
 private:
   // String with existence boolean.